Effect of superconductivity on spin dynamics in $\left({\mathrm{Y}}_{1-x}{R}_x\right){\mathrm{Rh}}_4{\mathrm{B}}_4$

An adiabatic—field-cycle method for ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ nuclear magnetic relaxation has been used to study the spin dynamics of rare-earth ions in dilute $\left({\mathrm{Y}}_{1-x}{R}_x\right){\mathrm{Rh}}_4{\mathrm{B}}_4$. Longitudinal dipolar fluctuations of rare-earth moments are found to be the main source for the spin-lattice relaxation time of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$. The variation of $T_1$ in the superconducting state is attributed to the reduction of the electronic spin-relaxation time ${\tau }_m$, which is mainly determined by the Ruderman-Kittle-Kasuya-Yosida (RKKY) interaction mediated by the conduction electrons. The remarkable reduction of the RKKY coupling below $T_c$ is discussed in connection with the reduction of the conduction-electron spin susceptibility $\chi \mathrm{}\left(q\right)\mathrm{}$.